In this seminar, I will discuss several hadronic interactions
by means of non-perturbative techniques based on unitarization
of amplitudes obtained from Chiral Perturbation Theory. First,
I will discuss the scalar sector, $J^{PC}=0^{++}$, for which one
needs to consider different meson-meson amplitudes. These are
obtained from the unitarization of chiral amplitudes, avoiding
{\it ad-hoc} parameterizations. We include the relevant channels
up to $\sqrt{s}=$ 2 GeV, so as to reproduce all the scalar
resonances appearing up to that energy. Later, I will consider
the scattering of scalar ($f_0,a_0$) and pseudoscalar ($\pi$,
$K$, $\eta$, $\eta'$) mesons, in order to study the possible
appearence of heavier pseudoscalar resonances. Given the
dynamically generated nature of the light scalars, these
amplitudes can also be obtained from Chiral Lagrangians.
Finally, I will elaborate further on the issue of the nature
of the $\sigma$ meson, by studying its quadratic scalar radius.
Given the small value found for the latter, it can be argued that
a four-quark picture for the latter is more appropiate than a molecular one.